The furnacemen are burning
their way through the clay plug in the tap hole. Molten iron will then
flow down the runners made in the sand on the cast house floor. They
will direct the flow of the molten iron using gates in the trough (foreground)
to iron ladles below the cast house floor.

In earlier times, the iron
would run down a large, sand lined trough (the sow) to hundreds of smaller
troughs (pigs) where the iron was allowed to cool. When it had solidified,
the pigs were broken from the sow and stacked. This is how pig iron
got its name.